Thermoplastic resin films such as a polypropylene film, a polyethylene terephthalate film and a nylon film are widely used as a packaging film because of excellent transparency, mechanical strength, processing aptitude and secondary processing property such as a bag-forming property.
For the purpose of providing the film described above with a gas-barrier function such as an oxygen-barrier property, a layer comprising a resin having a gas-barrier property such as a vinylidene chloride resin and a polyvinyl alcohol base resin is laminated on the surface of the above thermoplastic resin film.
However, the vinylidene chloride resin is excellent in a gas-barrier property but has a demerit on an incinerating property and a disposing property since it is a chlorine base resin. The polyvinyl alcohol base resin is excellent in an oxygen-barrier property in a dry state but has the problem that it is extremely reduced in an oxygen-barrier property under a high humidity by absorption of humidity.
Accordingly, devices such as cross-linking, modification treatment and combination with other compounds are tried. For example, a gas-barrier film prepared by providing a coating layer comprising a silica/polyvinyl alcohol base combined polymer on a thermoplastic resin film is disclosed in Japanese Patent Application Laid-Open No. 4563/1981. Further, disclosed in Japanese Patent Application Laid-Open No. 192454/1994 is a gas-barrier film prepared by providing a coating film comprising a combined product of metal alkoxide or a hydrolysate of metal alkoxide and a water-soluble resin having a hydroxyl group such as polyvinyl alcohol on a thermoplastic resin film.
However, the gas-barrier films described in Japanese Patent Application Laid-Open No. 4563/1981 and Japanese Patent Application Laid-Open No. 192454/1994 are improved in the problem that an oxygen-barrier property under a high humidity is extremely reduced by absorption of humidity, but the existing situation has been that the gas-barrier effect thereof is not satisfactory particularly under such high humidity as exceeding 90% RH.
Also, a gas-barrier film prepared by providing a coating film comprising a combined product of a water-soluble resin having a hydroxyl group such as polyvinyl alcohol, an inorganic stratified compound and a hydrolysate of metal alkoxide on a thermoplastic resin film is disclosed as a film which is further improved in a gas-barrier performance in the gas-barrier layer described above comprising the combined product in Japanese Patent Application Laid-Open No. 43219/2000. In such gas-barrier film, however, there has still been a room for improving a gas-barrier property under a high humidity and a gas-barrier property after boiling in hot water after providing a sealing layer.
Accordingly, an object of the present invention is to provide a gas-barrier film which shows a very excellent gas-barrier property even under such high humidity as exceeding 90% RH and which is excellent as well in a gas-barrier property after boiling.